1. Field of the Invention
This invention relates to a composite semiconductor device, and more particularly to a connection between a large-scale high density semiconductor substrate which is required to have high thermal conductivity and low thermal stress and a semiconductor package for protecting the same.
2. Description of the Related Art
In semiconductor devices such as IC, LSI, packaging is effected in order to protect a semiconductor substrate (which is hereinafter referred to as a semiconductor chip) having semiconductor elements formed therein from contamination sources such as dusts, chemicals, moisture which give bad influences and mechanical breakage. A package used for packaging is required to have air-tightness, good resistance to the high-temperature heating process in the assembling process, high mechanical strength, chemical stability, good electrical properties such as insulation and high-frequency characteristics, and the like. As the material of the package, synthetic resin, ceramic, glass, metal or the like may be used. When a ceramic package is used, a semiconductor chip (which is hereinafter simply referred to as a chip) is generally fixed on the inner bottom surface of the package which is open by use of adhesive agent of synthetic resin, and then a cap of ceramic, for example, is mounted on the opening portion of the package so as to hermetically seal the package. As the scale of the information processing device is increased and the operation speed thereof is enhanced it is more required to develop semiconductor devices suitable as the information processing device. In order to meet the above requirement, a multichip module (MCM) which is a composite semiconductor device is provided. The multichip module is constructed by a large silicon substrate (which is hereinafter referred to as a wiring substrate) of 50 mm square on which circuit wirings such as multi-layered wirings are formed and semiconductor chips such as a memory chip and CPU (central processing unit) fixed on the substrate. Since wirings between the chips are extremely short, a semiconductor device having high operation speed and high integration density can be obtained.
Next, mounting of a conventional MCM type semiconductor device is explained with reference to FIGS. 1A to 4. FIGS. 3 and 4 are a cross sectional view and a plan view of the semiconductor device after completion, FIG. 1B is a plan view of the semiconductor device, and FIGS. 1B and 2 are cross sectional views thereof. A package 3 is rectangular and a stepped portion 31 is formed in the inner peripheral portion thereof. Further, the package has connection wirings (not shown) formed therein and the wirings are exposed on the stepped portion 31. Electrode pins 32 for mounting the semiconductor device on a circuit board (not shown) such as a printed wiring board are formed in the peripheral portion of the package 3. The electrode pins 32 are connected to the connection wirings in the package. A heat radiation plate 9 is fixed on the rear surface of the package 3 by an adhesive agent 8. The wiring board 6 is fixed on the inner bottom surface of the package 3 by an adhesive agent 2. Further, chips 5 such as a CPU and memory element, for example, SRAM are mounted on the front surface of the wiring board 6 by an adhesive agent 4. In order to connect the chips 5 to each other, bonding wires 7 of aluminum or gold are formed to connect the chips 5 to the wiring board 6 and connect the wiring board 6 to the connection wires of the stepped portion 31 of the package. By this connection, the chips 5 are electrically connected to the external circuit board via the electrode pins 32.
The opening of the package 3 is hermetically sealed by use of a cap 10 of iron-series alloy such as Kovar (trade name of Westinghouse Co.). A coating (not shown) of gold, for example, is formed to cover the wirings formed on the wiring board 6 in an area of the wiring board 6 on which the chips 5 are mounted.
A method for manufacturing the composite semiconductor device is explained with reference to FIGS. 2 to 4. First, a silicone rubber-series adhesive agent 2 of low elasticity is coated on the entire surface of the wiring board 6. The adhesive agent 2 is set in contact with the inner bottom surface of the package 3, heated and pressed to be cured so as to fix the wiring board 6 on the inner bottom surface of the package. Next, a silicone rubber-series or epoxy-series adhesive agent 4 of high thermal conductivity is coated on each of the entire rear surface of the chips 5 and disposed on an area of the wiring board 6 on which the chip is mounted. Then, the adhesive agent 4 is heated and pressed to be cured so as to fix the chips 5 on the wiring board 6. Thus, the silicon semiconductor wiring board used in the composite semiconductor device (MCM) is made large (approx. 50 mm square) in order to permit a plurality of chips to be mounted thereon. As a result, after curing of the adhesive agent, warp and residual stress may occur in the wiring board 6 by a difference between the thermal expansion coefficients of the wiring board 6 and the package 3. Therefore, in order to reduce the warp and residual stress, an adhesive agent of low elasticity is used.
As described above, the wiring board used in the composite semiconductor device is large and most of the chips, for example, CPU disposed in contact with the wiring board generate a large amount of heat. Therefore, an adhesive agent of low elasticity for reducing the warp and residual stress caused by a difference between the thermal expansion coefficient of the wiring board and the thermal expansion coefficient of the package when the chip is mounted is used to fix the wiring board on the package and an adhesive agent of high thermal conductivity for radiating heat generated by the chips is used to fix the chips on the wiring board. However, in general, as the adhesive agent, epoxy resin or synthetic resin of low elasticity such as silicone rubber is used. In order to make the adhesive agent have a high thermal conductivity, it is necessary to introduce metal particles into the adhesive agent, but if the metal particles are introduced, the low elasticity of the adhesive agent is degraded. Therefore, it becomes impossible to obtain an adhesive agent which satisfies both of the high thermal conductivity and low elasticity at the same time. As a result, when the wiring board is fixed on the package in the prior art, an adhesive agent which satisfies both of the high thermal conductivity and low elasticity at the same time cannot be used.